Diagnostic assays have become an indispensable aid in medical and research fields for detecting a variety of components in biological fluids and tissue samples such as drugs, hormones, enzymes, proteins, antibodies, and infectious agents. A fundamental principle underlying the operation of a number of these assays is a specific recognition and binding reaction that occurs between two or more members to form a complex that can subsequently be detected. Normally, the members involve a capture reagent (e.g. receptor) that will specifically recognize and bind to a target analyte of interest (e.g. ligand) in a fluid test sample (e.g. whole blood, plasma, serum, urine, saliva, etc.). Moreover, a visually detectable indicator reagent is included in the reaction which will recognize and bind to any analyte complexed with the capture reagent to produce a signal indicating that a positive reaction has occurred. In particular, immunological assays are designed to function on the basis of antibody recognition and selective binding reaction to antigen and accordingly, have proven extremely valuable over the years in clinical applications for the detection of numerous infectious disease states.
In order to achieve accurate results, however, immunoassays often require precision in performing a series of time-consuming steps, as well as technical knowledge in operating sophisticated laboratory equipment. Accordingly, their use in diagnosing infectious disease has been essentially confined to clinical facilities that have the necessary resources for making such determinations including highly trained technical personnel and laboratories equipped with appropriate diagnostic equipment.
On this basis, as with many technologies, immunodiagnostic testing is evolving towards more simplistic approaches in the rapid identification and diagnosis of infectious disease states. The need for a simplistic qualitative assay for detecting analyte in a biological sample is becoming more desirable since it would offer an appealing possibility for use in less conventional settings having limited resources, e.g. physician's office, or domestic household. Whether in a public health clinic or a rural setting, it is preferable that an assay for detection of a target analyte in a fluid test sample be performed without the aid of complicated instruments and the requisite skills and knowledge of professionally trained personnel.
Another important factor to consider in pursuit of improved diagnostic testing is the lack of, or limited availability of, freezers and refrigeration in many third world countries. On this basis, it has become more desirable to develop assay reagents that will maintain their stability and integrity at room temperature for prolonged periods of time. Presently, some diagnostic devices and methods require the use of several assay reagents which have varying stability depending on the temperature at which they are stored and handled. Some of these reagents are stable at room temperature and may be stored for short periods of time, while others are relatively unstable and begin to deteriorate quickly, thereby adversely affecting the overall sensitivity and reliability of the assay. Thus, most commercially available diagnostic devices require at least one or more of the necessary reagents be kept at low temperatures in order to ensure their stability. Accordingly, a diagnostic device incorporating reagents that can be stored at ambient temperatures and remain stable for long periods of time while retaining all, or most, of its initial activity would have a clear advantage over current state of the art devices. On this basis, a factor worth considering towards simplifying diagnostic testing and thus, making it more practical and widely operational, is to minimize the number of assay reagents (e.g. mixing, washing, diluting solvents, etc.) and integrated steps in the assay protocol.
An immunodiagnostic assay which is simple to use, rapid and reliable would also be advantageous in improving screening and diagnostic services. According to the U.S. Center for Disease Control and Prevention report, rapid diagnostic tests enable healthcare providers to supply within minutes the test results to patients at the time of testing, thus potentially increasing the overall effectiveness of counseling and testing programs. It would also be expected that simplification of diagnostic devices and assays would likely be less costly to manufacture and perform compared to other conventional devices, thus making them economically feasible and more affordable to use in the interim. This is particularly desirable in third world countries where a simple, rapid, sensitive, and economical diagnostic device and assay would be ideal.
Towards this end, numerous analytical devices in an wide assortment of shapes, configurations and formats have been developed for detecting the presence of a target analyte in a fluid test sample, including chromatographic test strips, dipsticks, lateral flow and flow-through systems, to name a few. Many of these devices employ reaction membranes onto which a capture reagent capable of recognizing and binding to the target analyte is immobilized. In essence, the method of performing the assay typically involves applying a fluid test sample suspected of containing the target analyte, either directly or indirectly by filtration, to the reaction membrane. If the target analyte is present in the sample, it will bind to the capture reagent. Subsequent methods are then employed to determine whether the target analyte has bound to the capture reagent, thus indicating its presence in the sample.
U.S. Pat. No. 4,517,288 (Giegel, et al.) discloses methods for conducting ligand-binding assays using inert porous materials. In particular, the patent discloses immobilizing an immunological binding material (e.g. antibody) specific for the ligand of interest (e.g. antigen) within a finite test zone of the porous material and applying the ligand to the test zone which will be captured by the immobilized binding material. Immobilization of the binding material to the porous material may be achieved by any number of conventional methods including adsorption, covalent bonding, use of a coupling agent, etc. An enzyme-labeled indicator reagent, which will also recognize and bind with the ligand, is then applied to the test zone where it will become immobilized in an amount directly proportional to that of ligand present in the zone. A solvent is then applied to the center of the test zone to remove any unbound indicator reagent, thus enabling the determination of a signal to be made, with or without the aid of appropriate analytical instruments.
A more sophisticated version of a specific binding assay is described in U.S. Pat. Nos. 4,094,647, 4,235,601 and 4,361,537 (Deutsch, et al.) which incorporates a chromatographic test strip capable of transporting a developing liquid by capillary action. The test strip is designed so that it has a first zone for receiving a sample, a second zone impregnated with a first reagent capable of being transported by the developing liquid and a third zone impregnated with a third reagent. In addition, the device comprises a measuring zone and a retarding element which may be either the second reagent or the material of the strip. The first reagent is capable of reacting with one of the group consisting of (1) the sample, (2) the sample and the second reagent, or (3) the second reagent in competition with the sample, to form a product in an amount dependent on the characteristic being determined. A sample is contacted with the first zone and the strip is then dipped into the developing liquid to bring about transport of the sample and the first reagent to form the reaction product. The retarding element slows transport of either the product or the first reagent (the moving reagent) to spatially separate the two and the amount of the moving element is then measured at the measurement location.
A variation of the device by Deutsch, et al. is described in U.S. Pat. No. 4,960,691 (Gordon et al.) for the analysis of antigens, antibodies or polynucleotides which also uses a length of a chromatographic material (i.e. test strip), a solvent carrier and mobile reagents. Essentially, the strip has three separate zones comprising a first zone impregnated with a mobile reagent reactive with the analyte of interest, a second zone for receiving a test sample suspected of containing the analyte, and a third zone impregnated with an immobilized reagent which selectively binds to the analyte, thereby rendering the analyte in an immobilized form. Each zone is sequentially located an equidistant from its neighbour along a longitudinal axis of the test strip. The device optionally comprises fourth and fifth zones impregnated with indicator reagents that will provide a means of detecting the presence of the analyte. The method involves depositing the test sample in the second zone, followed by solvent addition to the strip at the end where the first zone is located so that sequential movement and arrival of the analyte and first reagent eventually occurs at the third zone. The site relationship between the second and third zones is such that the analyte is immobilized against solvent transport at the third zone prior to the first reagent reaching the third zone. Any interfering non-analyte sample components which are reactive with the first reagent are cleared from the third zone by solvent transport prior to the arrival of the first reagent to the third zone. Multiple and single pathway devices are also disclosed for accomplishing a variety of multi-step assay procedures.
U.S. Pat. No. 4,168,146 (Grubb, et al.) discloses the use of test strips for carrying out sandwich-type immunoassays. The strips are formed of bibulous carrier materials to which antibodies have been attached by adsorption, absorption or covalent bonding. Preferred test strip materials include cellulose fibre-containing materials such as filter paper, ion exchange paper and chromatographic paper. Also disclosed are uses of materials such as cellulose thin-layer chromatography discs, cellulose acetate discs, starch and three-dimensional cross-linked materials such as Sephadex (Pharmacia Fine Chemicals, Uppsala Sweden). The immunoassay is performed by wetting the test strip with a measured volume of a test sample suspected of containing the antigen. Any antigen present in the test sample migrates by capillary action along the test strip. However, the extent of migration of the antigen over a fixed time period is determined by the antigen concentration in the test sample because the bound antibodies retard the migration of the antigens for which they are specific. Afterwards, the antigen-containing areas of the diagnostic device are indicated by the addition of labeled antibodies.
An immunodiagnostic flow-through system comprising a series of method steps is disclosed in U.S. Pat. No. 4,632,901 (Valkirs, et al.). The first step involves taking a fluid test sample suspected of containing a first member of a specific binding pair (e.g. antigen) and pouring it onto a porous material to which a second member of the specific binding pair (e.g. antibody) is immobilized. Influenced by the capillary action properties of an absorbent material, the fluid test sample is drawn downwards in a vertical direction through the porous material and pass the immobilized antibody. Any antigen present in the sample will subsequently be captured by the immobilized antibody. The second step involves passing a separate solution of labeled antibody through the porous material so that the labeled antibody may bind to the antigen already captured by the immobilized antibody to form a three-membered complex. Any unreacted or unbound labeled antibody is then flushed away from the porous material via a third step, normally referred to as a washing step, using a suitable reagent which may then be followed by an incubation period. Finally, a fourth step involving a separate solution containing a substrate reactive with the label on the antibody of the second solution is added to cause a visible color change indicative of the presence of the antigen of interest. To facilitate accurate performance of this method, the apparatus is designed in such a way as to funnel the sample through to the absorbent material which, by capillary action, draws the sample through the material and into the bottom of the apparatus.
An immunodiagnostic flow-through system described by Liotta in U.S. Pat. No. 4,446,232 utilizes a combination of two different reaction zones arranged in three separate layers. The first reaction zone comprises two layers fabricated from porous material wherein the first and second layers are impregnated with soluble enzyme-linked antibody and immobilized antigen, respectively. The third layer, or second reaction zone, contains immobilized indicator reagent that will react with the enzyme linked to the antibody of the first reaction zone to produce a color. If a liquid sample contains the antigen of interest, then after the sample is applied to the first reaction zone, the antigen contained therein will bind with the soluble enzyme-linked antibody and diffuse through to the second reaction zone following a short incubation period. The presence of antigen will be detected when the enzyme reacts with the indicator reagent to produce a color. By contrast, if a liquid sample does not contain any antigen, then the enzyme-linked antibody will migrate to the second layer of the first reaction zone, aided by diffusion of the fluid test sample, where it will bind to immobilized antigen. The binding reaction that occurs will prevent any enzyme-linked antibody from reaching the second reaction zone where it would react with the indicator reagent. Thus, in this particular scenario, no color is observed indicating the lack of antigen in the fluid test sample.
While the methods and devices described above may provide compact and somewhat reliable means for performing immunodiagnostic assays, several problems regarding their use still exist. In particular, one of the disadvantages encountered in determining the presence or absence of a target analyte in the majority of cases is the requirement to perform several addition and washing steps using a range of solvents. The washing steps are essential at various stages of the assay protocol in order to prevent undesired cross-reactions and to remove any excess unbound reagents and substances which may subsequently interfere with the results. Unfortunately, this only complicates the overall procedure and effectively reduces the level of efficiency desired in order to develop an improved and simplified version of an immunodiagnostic assay. Thus, the need to adhere to several addition, washing and incubation steps has largely limited these procedures to clinical settings where skilled personnel and sophisticated equipment are available to carefully monitor and perform the assay with precision and accuracy.
In addition, immunodiagnostic assays that employ chromatographic test strips or dipsticks suffer from a problem regarding sequential treatment with one or more solvents at various stages of the assay procedure. As each solution is added to the device, or as each device immersed into successive solutions, the opportunity for spillage or contact between the solutions and the user are enhanced, thus leading to possible contamination and reduction in the reliability of the test.
Depending on the assay and device used, it is usually necessary that the test sample be diluted with an appropriate reagent prior to application so that it will diffuse more easily throughout the porous material and/or not overwhelm the concentration of the labeled reagent. However, dilution of the test sample not only reduces the speed and ease of performing an assay by including an additional step and reagent, but it can also reduce the sensitivity of an assay due to the correlation of analyte concentration to the detection signal generated.
A further disadvantage associated with the use of some immunodiagnostic devices, particularly those incorporating lateral-flow techniques, is that they characteristically require long incubation periods at various stages of the procedure. Depending on the relative mobility of the analyte of interest, the type of reagents and solvent used, and the site relationship between the different reaction zones, adequate time is essential in order to allow for efficient migration of all the various components along the chromatographic solid phase material. Moreover, the lateral flow technique often contributes to a higher incidence of inaccurate results due to the tendency of mobile reagents to accumulate at, rather than clear, the periphery of the reaction zone. As a result, these reagents will often interact at the zone and produce color products that may be easily mistaken for a true positive or negative result.
Accordingly, the present invention provides an improved rapid diagnostic device, assay and multifunctional buffer for the detection of a target analyte in a fluid test sample which is efficient, reliable and practical to perform. The simplified 2-step assay utilizes a multifunctional buffer reagent and a dual component flow-through device comprising a test unit in combination with a detachable post-filter unit which are capable of receiving the fluid test sample and multifunctional buffer, respectively.
The multifunctional buffer serves as a combination washing, diluting, wetting and resolubilizing reagent, without sacrificing the sensitivity or specificity of the diagnostic assay. Additionally, the buffer is formulated to preserve and optimize protein stability, as well as minimize, if not eliminate, non-specific interactions that might lead to the generation of a false signal.